1. Statement of the Technical Field
The invention concerns antennas and more particularly aperture coupled antennas that can be dynamically modified to operate over a relatively large bandwidth.
2. Description of the Related Art
Patch antennas are well known in the art and are used in a wide variety of applications. They can be manufactured in a nearly unlimited number of shapes and sizes, and can be made to conform to most surface profiles. Patch antennas also possess an omni-directional radiation pattern that is desirable for many uses.
One negative aspect of patch antennas is that they usually have a relatively narrow impedance bandwidth. For a typical classically fed patch antenna, bandwidth is usually about 2% to 3%. Patch antennas that are fed with an aperture or slot can have slightly higher bandwidths, in the range from about 4% to 6%, but this is still too narrow for many applications. The impedance of a patch antenna is also noteworthy as it can depart significantly from 50Ω. Consequently, most patch antennas need a matching network in order to ensure efficient power transfer, particularly if when fed with coaxial cables that can be lossy at high levels of VSWR.
Impedance matching for a patch antenna can be accomplished using several different approaches. For example, a quarter wave high impedance transmission line transformer can be used for this purpose. Alternatively since the impedance is at a minimum at the center of the patch and increases along the axis, a 50Ω microstrip line can be extended into the interior of the patch to achieve a suitable match. In yet another alternative, a center conductor of a coaxial line can be routed through a dielectric substrate on which the conductive patch is disposed to contact the underside of the patch at a selected impedance point.
Still, the performance of most conventional matching systems will be frequency dependent. Accordingly, the input impedance of the antenna system will tend to vary considerably over a relatively large bandwidth. Consequently, the usable bandwidth of the conventional patch antenna will remain relatively limited.